Determine which areas of research hold your interest. The amount of time it takes to complete a dissertation varies from field to field, but you can count on having a few dark months or even years of despair if your research does not go smoothly. Be sure you are working on a topic that you feel is relevant and worth your efforts.
Calculate the costs of this topic. This should be done on several levels. How much time will this topic take to complete? Will it require research abroad or significant grant funding? Can your significant other manage the stress that this research will put on your relationship?
Conduct a thorough literature review. Has this topic been done before? If not, why not? Are there major scholars who oppose this line of inquiry and why? This, incidentally, may make some topics more appealing, not less.
Sketch out the basic parameters of the research to determine if there will be sufficient scholarly interest in the topic to publish your findings. Count on producing at least one scholarly article while your dissertation is in process, so that you will have something in print when you go on the academic job market. Is this topic likely to yield more articles, perhaps a book?
Consult with faculty advisers regarding your topic. You'll need an adviser who's committed to you and your work, and who's willing to manage a committee. You may have to adapt your topic slightly to ensure that this faculty member remains interested, but don't stray too far from your own personal interests.